Processes have been disclosed in the prior art for the removal of sulfur dioxide from waste gases (U.S. Pat. No. 5,344,529) and for the removal of nitric oxide from waste gases (U.S. Pat. No. 4,925,639). These processes have in common the use of a column packing that is electrically conductive. Such packing not only serves as a surface for absorption of sulfur dioxide or nitric oxide but also enables the simultaneous electrolysis of the dissolved gases. In this manner these processes overcome the limited solubility of these gases.
Notwithstanding the advantages of the prior art, certain drawbacks have been encountered. In the case of the removal of sulfur dioxide from waste gases and its recovery as sulfuric acid, a bipolar process using conductive packing was disclosed. This loose packing material is dumped into a column that contains two electrical contacts. As current is passed through the column packing, the pieces in the column take on positively and negatively charged sites at which the desired electrochemical reactions occur.
The effectiveness of the bipolar process will depend in part on the current efficiency. Should packing material with high electrical conductivity be used, the packing will tend to act like a short circuit, and limited electrolysis will take place. This problem is illustrated by the use of packing fabricated from nickel base superalloys. This material has an electrical resistivity of 133 micro ohm-cm compared with 1.35 ohm-cm for 30 percent sulfuric acid. Thus the conductivity of the packing is close to 10.sup.4 greater than the acid.
Even using graphite as a packing material, the bed will function as a shunt. Having a resistivity at about 1400 micro ohm-cm, graphite will show only marginal improvement over the most promising nickel alloys. As a guiding principle, the distribution of current in parallel electrical circuits is inversely proportional to the resistivities.
The process for nitric oxide removal avoids the problem of the packing conductivity by using the packing only as the anode. The cathode is located externally to the column. The efficiency of this process suffers, however, from the electrical resistance of the acid in the lines which connect the column to the compartment which contains the cathode.
It is therefore an object of the present invention to overcome the limitations of the prior art, particularly with respect to the electrical efficiency.
At the same time it is an object to provide a means that preserves the improvements of the prior art including reduced acid circulation, high scrubbing efficiency, and compact design.
These and other objects, features and advantages of the invention will be apparent from the accompanying drawings and the following description.